1. Field of the Invention
This invention relates to motion picture film projectors such as the type commonly used in motion picture theaters.
2. Description of Related Art
The continuing popularity of motion pictures has created a demand for improved and lower cost projection systems for cinemas as well as for home theaters. Conventional film projectors are costly to purchase, and expensive to operate and maintain. Conventional film is expensive, and unfortunately the film itself wears out too quickly. Eventually, film may be replaced by entire electronic projection systems that require only a stream of data; however, the near-universality of film at the present time ensures that the demand for conventional projectors will continue at least in the near future. Therefore, it would be useful to improve the film projections systems, as well as to provide an electronic alternative. In general, desired improvements to projection systems include higher resolution, lower cost of operation and maintenance, and accurate, and consistent color over time.
Conventional film projectors, such as shown in FIG. 1, include a broadband source of incoherent light such as a projection lamp, a parabolic reflector, a heat (infrared) filter, and an ultraviolet (uv) filter. The incoherent light is provided to conditioning optics that illuminate an aperture, and a film delivery system advances film over the film aperture where each frame of the film is momentarily stopped and illuminated for display. For example, some conventional projectors operate at 24 Hertz; i.e. 24 frames are stopped and displayed each second. A shutter in the aperture blanks the frame (i.e. blocks illumination of the aperture) while the film is moving across the aperture. Furthermore, in order to prevent the appearance of flicker to a moviegoer, the shutter operates at twice the frequency of the frame display; e.g. if the frame display operates at 24 Hz, then the shutter operates at 48 Hz. In other words, while the film is stopped in the aperture, the shutter opens and closes twice. Projection optics project the xe2x80x9cmovingxe2x80x9d image from the illuminated aperture onto a screen. In a typical installation, all components of the film projector are mounted on a console.
The projection lamp is typically a Xenon arc lamp that is coupled to a specially coated parabolic reflector to dissipate heat and uv radiation and focus xe2x80x9cwhitexe2x80x9d light from the Xenon lamp onto the film aperture. High intensity Xenon lamps operate continuously, and emit only about 50% of their energy as usable, visible light; the remaining energy is emitted as infrared and ultraviolet radiation. Given a typical Xenon lamp rating of 3,000 watts, a tremendous amount of waste heat and uv radiation is produced. Although most of the waste heat is filtered out by the heat filter, a small fraction leaks through, enough to damage the film. Furthermore, a small fraction of the uv radiation leaks through the uv filter, and this ultraviolet radiation rapidly degrades the dyes in the film, resulting in a loss of color fidelity in a short time. In other words, non-visible radiation produced by high intensity arc lamps effectively degrades and damages a film print and drastically shortens its useful lifetime. The parabolic reflector, which reflects the high intensity light from the flashlamp, also suffers damage from non-visible radiation and is costly to replace.
Of the 50% usable light emitted by a Xenon lamp in a typical motion picture projector, only a small amount of this percentage, typically less than 10%, actually can be used to illuminate the film aperture, in part due to optical losses and also due to emission by the lamp in all directions. Furthermore, the film typically absorbs about 25% of the incident light, leaving less than 8% of the visible light for projection on the screen. Therefore, typically less than about 5% of the total power of a Xenon lamp is utilized to illuminate the film, and less than about 4% of the total power is actually projected onto the screen. Thus, over 95% of the light emitted by the Xenon lamp is waste energy, much of it in the form of heat that must be dissipated in some way.
Electronic projectors have been proposed to replace film projectors. Electronic projectors are xe2x80x9cfilm-lessxe2x80x9d, requiring only a stream of data (in an electronic form such as digital or analog) to generate a high resolution image, instead of the series of physical images embedded in film. However, electronic projectors for cinemas are in their early stages of development and implementation, and are not yet widely accepted. To promote acceptance of electronic projection systems in cinemas that traditionally have relied upon film projectors, it would be advantageous if the same projector could accommodate either film or electronic images.
Modulated laser projection systems, such as shown in FIGS. 2 and 3, are electronic projectors that have been proposed to replace film projectors. A laser emits a highly directional beam of radiation at a predetermined wavelength. For laser projection systems, a red laser, a green laser, and a blue laser are used to represent the full spectral range to the human eye. The lasers (or the beams from the lasers) are modulated in accordance with electronic image information, and then projected to provide the appropriate color at each pixel of the image. Some laser display systems modulate and then scan the laser beams, while other laser display systems directly modulate the beam imaged onto the screen.
A dual mode laser projection system is disclosed that has a film mode and an electronic mode, either of which can be used for projecting an image onto a screen. In the film mode an image is projected by illuminating film with lasers, and in the electronic mode an image is created by modulation of lasers with data to project an electronic image. Thus, the same projector can be used like a film projector to project film images, or it can be used to project an electronic image by modulation of the lasers using electronic means.
The laser projection system comprises a laser source that provides light having a plurality of colors, projection optics, a film module that includes a film aperture and a film delivery system that advances film over the film aperture, and an electronic module that includes a laser beam modulation system. In the film mode the film module is situated to receive the light from the laser source, and the film aperture is configured to be illuminated by the light, thereby to supply an image to the projection optics for projection onto the screen. In the electronic mode, the electronic module is situated to receive the light from the laser source, and supply modulated laser light to the projection optics to project an image onto the screen.
In some embodiments, the film aperture includes a system that momentarily stops each frame for illumination, and in the film mode, the laser source is synchronized with the film aperture so that the laser source is modulated by the laser beam modulation system to illuminate the film aperture substantially only while a frame of the film is stopped. In other embodiments, the aperture comprises a shutter. The laser source may comprise a plurality of lasers each providing a different color, including a red laser, a green laser, and a blue laser. In some embodiments, the laser source comprises a plurality of Laser-CRTs, each providing a different color, including a red Laser-CRT, a green Laser-CRT, and a blue Laser-CRT.
One advantage of using a laser source to illuminate film is that lasers primarily emit highly directional, visible laser radiation at a single wavelength, thereby avoiding emitting harmful radiation (e.g. uv radiation) and heat, which would otherwise damage the film, thereby extending the film""s lifetime and quality. Furthermore, the laser projection system is substantially more energy efficient than Xenon lamp-based system.
In some embodiments, the color temperature of the light illuminating the film can be selected by using a color temperature control system to increase and/or decrease the relative light intensity contribution of each of the lasers. The ability to select color temperature could be useful, for example, to allow a filmmaker the artistic freedom to select a color temperature for illuminating the film, or different color temperatures for different parts of the film. Furthermore, the viewing experience would be consistent for all viewers because the color temperature would be closely controlled. In summary, the ability to control color temperature provided by color temperature control system allows color flexibility for filmmakers, while providing color consistency for moviegoers.